Method for characterizing hiding of coating compositions and apparatus used therefor

ABSTRACT

The present invention is directed to an apparatus and a method that characterizes the hiding of coating compositions, such as automotive OEM and refinishes paints. The method is directed to sequentially storing in a computing device hiding data obtained by measuring the color difference (ΔRGB) in reflections of light and dark portions of target areas of a monotonic coating resulting from a coating composition applied over hiding test panel and by measuring coating thicknesses of the monotonic coating that correspond to the target areas, classifying the type of the hiding data, selecting fitting equations applicable to the classified hiding data, fitting the selected equation to match the classified hiding data and locating a hiding thickness on the monotonic coating that corresponds to the threshold value of the color difference at that location to determine the hiding thickness of the coating composition.

FIELD OF INVENTION

The present invention is directed to an apparatus and a method used forcharacterizing hiding of coating compositions, such automotive paints.

BACKGROUND OF INVENTION

The hiding of a pigmented coating composition, such as an automotive OEM(original equipment manufacturer) paint or automotive refinish paint, isgenerally characterized by determining its hiding power, usually byvisual observation. The hiding power is the measure of a coatingcomposition's ability to cover a surface opaquely so that an underlyingcoating, such as that from a primer, cannot be seen in visible light.The absorption and scattering of incident light affects the hiding powerof a coating composition. Thus, for example, the compositions of darkercolors absorb more intensely than those of lighter colors and hence suchcompositions have greater hiding power than those of lighter colors.

Several methods for determining the hiding power of a coatingcomposition are known in the coatings art. One such method (ASTM D6762-02a) supplied by ASTM International, West Conshohocken, Pa.)consists of applying a monotonic, i.e., wedge shaped, layer of a coatingcomposition over a test hiding pattern affixed to a hiding panel. Afterthe applied layer cures or dries into a coating, then visually observingthe coating under light at an angle perpendicular to the coating todetermine a position on the coating where the test pattern is barely orno longer visible. The aforementioned test pattern is typically in theform of two abutting white and black stripes and the monotonic layer ofincreasing gradient is thinnest at one end of the test pattern andthickest at the other end of the test pattern. The monotonic coating istypically produced by progressively increasing the number of sprayingpasses over one end of the panel to the other. Typically, on one end ofthe panel, the test pattern is clearly visible through the coating(non-hiding end) whereas the test pattern is not visible at other end(hiding end). The hiding power of a coating composition is the lowestcoating thickness at which hiding occurs. That would be the coatingthickness at which the coating, such as paint, should be applied.Anything less will not be adequate and anything more would be a waste.The process for measuring the hiding power of a coating composition iscurrently done by technicians who visually examine the hiding panel andmark the position where the hiding pattern is no longer visuallydiscernable. A film thickness gage is then used to measure the coatingthickness at this position on the panel and recorded as the hiding filmthickness for that particular coating. This approach is prone tosignificant errors because of visual subjectivity, due to variations inlighting used for observing the panel and observing geometry variation.Furthermore, the film thickness measured by a gage is very sensitive tothe technique of usage and adds another significant source of error.

Some prior art references describe instruments for measuring colordifference between two areas and when this difference goes below apredetermined value, the coating thickness at that point would representthe hiding thickness of that coating composition. However, using thesame pre-determined value to represent hiding for different colors andfinishes does not produce the best results. For finishes that have highsparkle from metallic flakes, hiding occurs even with relatively highcolor difference whereas for solid colors, such as certain whites andyellows, hiding only occurs at extremely low values of color difference.Thus, using a single predetermined value does not produce accurateresults. Furthermore, color and film thickness measurements on typicallyprepared hiding panels can have various random errors in them. Airbubbles under the hiding test pattern sticker, smudges and scratches,etc., can result in incorrect readings of color intensities. Similarly,the coating thickness measurements at any one point can have errors.Thus, most of the known methods fail since they have the aforedescribederrors. Thus, a need exits for a method and hiding measurement apparatusthat is practical in that it is adaptive and error correcting and itaccurately determines the hiding of various colors and coatingcompositions, including those that contain flakes, such as metallic,pearlescent, and mineral flakes

The patent publication GB 1404 636 describes a system and a method fordetermining the hiding power of paints. According to the patentpublication, a layer of paint is applied on a substrate with black andwhite regions. The substrate is illuminated with light, and the lightreflected from the black and white regions of the substrate is capturedby photoelectric cells. Potential difference measured by usingphotoelectric cells is considered to be proportional to the differencein reflective value of the black and white regions. Hiding power ispresumed to be a function of the difference in the reflectance over theblack and white regions. However, the patent publication only mentions asystem and method for determining hiding power of paint. It is notdirected to determining the hiding thickness at which the hiding occurs.Therefore, a need still exists for more accurately and consistentlydetermining the hiding thickness of coating compositions, even on wetpainted substrates.

STATEMENT OF INVENTION

The present invention is directed to a method of characterizing hidingof a coating composition, said method comprising:

(i) applying a monotonic layer of said coating composition over a testpattern affixed to a hiding test panel to produce a monotonic coatingthereon;

(ii) sequentially uniformly illuminating target areas P₁ to P_(n) ofsaid monotonic coating, each said target area comprising a light portionand a dark portion;

(iii) sequentially directing reflections of said target areas P₁ toP_(n) to a photosensitive device for acquiring:

-   -   (a) intensities Ir₁ to Ir_(n), Ig₁ to Ig_(n) and Ib₁ to Ib_(n)        of said light portions of said areas P₁ to P_(n), and    -   (b) intensities dr₁ to dr_(n), dg₁ to dg_(n) and db₁ to db_(n)        of said dark portions said areas P₁ to P_(n);

(iv) sequentially measuring applied measured thicknesses X₁ to X_(n) ofsaid monotonic coating at locations that respectively correspond to saidtarget areas P₁ to P_(n);

(v) sequentially computing measured Y₁ to Y_(n) at said target areas P₁to P_(n) by using the formula:

[(Ir_(i)−dr_(i))²+(Ig_(i)−dg_(i))²+(Ib_(i)−db_(i))²]^(0.5)  (1)

wherein i ranges from 1 to n, and said measured Y₁ to Y_(n) are measuredΔRGBs;

(vi) storing on a computing device hiding data comprising said measuredY₁ to Y_(n) and said applied thicknesses X₁ to X_(n);

(vii) determining a threshold measured Y_(th) by using the formula:

measured Y _(th)=Log_(e) (measured Y _(max)),  (2)

said measured Ymax being the maximum value within the range of saidmeasured Y₁ to Y_(n);

(viii) sequentially comparing said measured Y₁ through Y_(n) to identifyfirst measured Yq that is less than measured Y_(th) wherein q fallswithin said range 1 to n;

(ix) computing a ratio (q/n) to classify said hiding data, wherein saidis classified as:

-   -   (a) type 1 hiding data when said ratio is in the range of 0.01        to less than 0.25,    -   (b) type 2 hiding data when said ratio is in the range of 0.25        to less 0.35,    -   (c) type 3 hiding data when said ratio is in the range of 0.35        to less 0.50, or    -   (d) type 4 hiding data when said ratio is in the range of 0.50        to 1.00;

(x) selecting one or more fitting equations applicable for said type 1hiding data, type 2 hiding data, type 3 hiding data or type 4 hidingdata, wherein said fitting equations define a relationship between (y)and (x), said (y) being a fitted color difference that corresponds tosaid (x), which is a fitted thickness on a fitted curve generated bysaid one or more fitting equations;

(xi) fitting said one or more selected fitting equations to match pairedmeasured (X₁, Y₁) to paired measured (X_(n), Y_(n)), wherein said fittedcurve has a fitted baseline value of y_(b) at an asymptote of saidfitted curve;

(xii) selecting a fitted threshold value y_(th) above said fittedbaseline value y_(b), wherein said fitted threshold value y_(th) issuited for said type 1 hiding data, type 2 hiding data, type 3 hidingdata or type 4 hiding data; and

(xiii) locating a hiding thickness x_(h) of said coating compositionthat corresponds to said fitted threshold value y_(th) on said fittedcurve.

The present invention is also directed to an apparatus forcharacterizing hiding of a coating composition, said apparatuscomprising:

(i) a light source for illuminating target areas P₁ to P_(n) of a hidingtest panel at a desired angle of incidence and light source intensitywherein each said target area comprises a light portion and a darkportion, said hiding test panel having a monotonic coating from saidcoating composition applied thereon;

(ii) a first motion translating system affixed to a bed of saidapparatus, said first motion translating system comprising a firstmovable stage and a first mechanism for translating said first movablestage;

(iii) a fixture affixed to said movable stage to position said hidingtest panel thereon;

(iv) a photosensitive system affixed to said bed of said apparatus, saidphotosensitive system being positioned to receive reflections of saidlight portion and said dark portion of each said target area of saidhiding test panel;

(v) a second motion translating system affixed to said bed of saidapparatus, said second motion translating system comprising a secondmovable stage and a second mechanism for translating said second movablestage in a direction perpendicular to that of said first movable stage;

(vi) a coating thickness detector affixed to said second movable stagefor measuring thicknesses X₁ to X_(n) at locations that respectivelycorrespond to said target areas P₁ to P_(n); and

(vii) a computing device connected to said light source, saidphotosensitive system, said first and second motion translating systemsand said coating thickness detector to direct steps performed by saidlight source, said photosensitive system, said first and second motiontranslating system and said coating thickness detector in accordancewith a computer readable program code means stored in said computingdevice.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 broadly illustrates one of the embodiments of the apparatus ofthe present invention.

FIG. 2 provides the schematic presentation of how the various componentsof the apparatus of the present invention interact with a computingdevice.

FIG. 3 is a plan view of a hiding test panel on which a hiding patternhas been mounted.

FIG. 4 is a cross-sectional view of the hiding test panel of FIG. 3taken along cross-section 4-4 in FIG. 3.

FIG. 5 is a plan view of a hiding test panel of FIG. 3 that has beencoated with a monotonic coating.

FIG. 6 is a cross-sectional view of the coated hiding test panel of FIG.5 taken along cross-section 6-6 in FIG. 5.

FIG. 7 is a cross-sectional view of the hiding test panel of FIG. 3taken along cross-section 7-7 in FIG. 5.

FIG. 8 represents a flowchart that broadly illustrates and providesdetails of means for configuring computer readable program code means inthe computing device for operating the apparatus of the presentinvention.

FIGS. 9 to 13 are various graphical renditions of ΔRGB (color differencebetween light and dark portions of the hiding pattern applied over thehiding test panel) versus the coating thickness of the monotonic coatingapplied over the test panel.

DETAILED DESCRIPTION OF PREFERRED THE EMBODIMENT

As defined herein:

Hiding test panel means a substrate over which the test pattern ispreferably centrally affixed, such that a bare surface on the hidingpanel is exposed on one or both sides of the test pattern. Substrate canbe made of any conventional substrates, such as steel, aluminum, copper,wood, glass or plastic resin.

As shown in FIGS. 1 and 2, major components of an apparatus 1 of thepresent invention for characterizing hiding of a coating compositionincludes a bed 2 having positioned thereon a light source 4, a firstmotion translating system 6, a photosensitive system 8, a secondtranslating system 10, a coating thickness detector 12 and aconventional computing device 14, such as a Optiplex® GX620 Minitowercomputer supplied by Dell Computers of Round Rock, Tex. As shown in FIG.2, computing device 14 is in communication with light source 4,photosensitive system 8, first motion translating 6 and second motiontranslating 10 systems and coating thickness detector 12. Computingdevice 14 is conventionally programmed in accordance with a computerreadable program code means stored in computing device 14 to directsteps performed by light source 4, photosensitive system 8, first motiontranslating 6 and second motion translating 10 systems and coatingthickness detector 12.

First motion translating system 6 is preferably affixed to bed 2 andincludes a first movable stage 16 which can be translated by a firstmechanism 18 for translating first movable stage 16 in one direction,preferably in a horizontal direction. First motion translating system 6can be any suitable conventional system, such as Model Number MA2515supplied by Velmax, Inc. of Rochester, N.Y.

FIG. 3 illustrates a typical hiding test panel 20 shown before it iscoated. Test panel 20 can be made of any suitable substrate used formeasuring hiding, such as metal, wood, glass, stone, fabric, or plastic.Metal substrate is preferred. Test panel 20 can have different shapes,such as rectangle, square, circle, oval, triangle, or irregular.Rectangular shape is preferred. Test panel 20 can be a flat surface, acurved surface, or a spherical surface. Flat surface is preferred. Morepreferably, test panel 20 is a 10.1 cm×30.5 cm (4″×12″) flat rectangularmetal plate. Test panel 20 is typically provided with a test pattern 22,which is typically a pattern with a pair of abutting stripes ofcontrasting colors, such as a dark stripe 22A and a light stripe 22B.Dark stripe 22A can be of any dark a color, such as black, red, or otherdark colors. Black color is preferred. Light stripe 22B can be of anylight color, such as white, gray, or other light color. White color ispreferred. Dark and light stripes 22A and 22B are normally arranged as apair such as, a black stripe abutting a white stripe, a black stripeabutting a gray stripe, or a red stripe abutting a gray stripe. Testpattern 22 is preferably centrally positioned on test panel 20 to exposea bare surface 22C on one or both sides of test pattern 22. A detaileddescription of a typical hiding pattern is provided in ASTM D 6762-02a(ASTM International in West Conshohocken, Pennsylvania 19428).

As shown in FIGS. 5 and 6, a monotonic layer from a coating compositionbeing tested for its hiding characteristic is conventionally applied,such as by spraying test pattern 22 on test panel 20. Upon cure, themonotonic layer applied over hiding panel 20 results into a monotoniccoating 24, which is a wedge-shaped coating having a substantially everincreasing thickness that increase but never decreases from one end oftest pattern 22 applied over hiding panel 20 to the other end of testpattern 22. Test pattern 22, typically made of an appropriately coloredpaper, film or coating that is adherent to the surface of hiding panel20, would have some thickness, which can adversely affect themeasurement of the coating thickness by apparatus 1. To eliminate suchan error, monotonic coating 24, as shown FIGS. 5 and 6, also extendsover locations 26 on exposed test panel surface 22C that are outside andalongside the surface of hiding panel 20 covered by test pattern 22. Asshown in FIG. 1, hiding panel 20 is positioned in a positioning fixture28 affixed to movable stage 16, preferably by providing panel 20 withnotches in which matched detents located on fixture 28 can lock panel 20firmly in place during the hiding measurements. It is contemplated thatother suitable securing means for firmly securing panel 20 to fixture28, such as magnets, clamps or a bezel frame are equally suitable foruse in the present invention.

As seen in FIGS. 1, 2 and 5, light source 4 is used to uniformlyilluminate target areas P₁ to P_(n) of monotonic coating 24 on hidingtest panel 20 at a desired angle of incidence and light sourceintensity. Target areas P₁ to P_(n) are those areas on monotonic coating24 that are sequentially illuminated by light source 4, viewed byphotosensitive system 8 and analyzed by computing device 14. By way ofexample, a 10.1 cm×30.5 cm (4″×12″) hiding test panel 20 can have 10 to30 target areas, i.e., where n can range from 10 to 30 (P_(n)) at 1.27cm (½′) intervals by sequentially translating first movable stage 16 bymeans of first mechanism 18 at about 1.27 cm (½′) intervals to exposetarget areas P₁ to P_(n) on monotonic coating 24. As shown in FIG. 5,each target area would include a light portion 30 and a dark portion 32that results from the underlying dark stripe 22A and light stripe 22B.Light source 4 can be any suitable conventional light source, such asSoLux® 40 watt, 4700 degree Kelvin, 17 degree spot solar simulator lampsupplied by, Tailored Lighting Inc. of Rochester, N.Y.

An adjustable post 7, which is preferably affixed to bed 2 is providedwith means to secure photosensitive system 8 to sequentially receivereflections of paired light portion 30 and dark portion 32 of eachtarget area P₁ through to P_(n) on monotonic coating 24. One of suchsuitable photosensitive system 8 includes a video camera, such as modelnumber GPUS522 supplied by Panasonic Systems Solutions Company ofSecaucus, N.J. Preferably, photosensitive system 8 is positioned at 90degrees to hiding test panel 20.

The distance between light source 4 and target areas P₁ to P_(n) onmonotonic coating 24 and the angle of incidence of light emanating fromlight source 4 typically control the uniformity of the illumination thetarget areas P₁ to P_(n). Light source 4 is preferably positioned at anangle ranging from 5 degrees to 60 degrees to hiding test panel 20. Theuniformity of illumination over target areas P₁ through to P_(n) can beobtained by controlling the alignment of photosensitive system 8,monotonic coating 24 and light source 4. The aforedescribed alignmentcan be attained by providing various components of apparatus 1, suchlight source 4 and photosensitive system 8 with fine tuning adjustmentmeans, such as micro-threaded set screws (not shown).

As seen in FIGS. 1, 2 and 5, second motion translating system 10 ispreferably affixed to bed 2 apparatus 1. Second motion translatingsystem 10 typically includes a second mechanism 11 for translating asecond movable stage 13 in a direction perpendicular to that of firstmovable stage 16. Second motion translating system 10 can be anysuitable conventional system, such as Model Number MA2509 supplied byVelmax, Inc. of Rochester, N.Y. A gage head 9 of a coating thicknessdetector 12 is affixed to a second movable stage 13 for measuringthicknesses X₁ to X_(n) at locations 34 that respectively correspond topaired light portion 30 and dark portion 32 on each of target areas P₁through to P_(n). One of suitable coating thickness detector 12 that canbe used in the present invention is CMI-213 Film thickness gage suppliedby Oxford Instruments of Elk Grove, Ill.

As shown in FIGS. 5, 6, 7 and 8, computer readable program code means100 of apparatus 1 of the present invention include means 102 forcontrolling first motion translating system 6 to sequentially directreflections of light portion 30 and dark portion 32 of target areas P₁to P_(n) of monotonic coating 24 to photosensitive device 8 to acquirecolor intensities Ir₁ to Ir_(n), in red color Ig₁ to Ig_(n) in greencolor and Ib₁ to Ib_(n) in blue color of light portions 30 of areas P₁to P_(n), and intensities dr₁ to dr_(n), dg₁ to dg_(n) and db₁ to db_(n)of dark portions 32 of areas P₁ to P_(n) on monotonic coating 24. Thesevarious color intensities, so called RGB (red, green and blue)intensities, are obtained by using appropriate red, blue and greenfilters in photosensitive system 8.

Means 100 can further include means 104 for controlling second motiontranslating system 10 to sequentially direct gage head 9 of a coatingthickness detector 12 for measuring thicknesses X₁ to X_(n) at locations34 that respectively correspond to said target areas P₁ to P_(n),typically on one or the other side of pattern 22.

As shown in FIG. 8, computer readable program code means 100 can includemeans 106 for controlling time of exposure of a photo sensitive surfacein photosensitive device 8 to the reflections of light portion 30 anddark portion 32 of target area P₁ to attain highest obtainable contrastbetween light portion 30 and dark portion 32 of target area P₁ withoutsaturating the photosensitive device anywhere else on the panel.Position P₁ of monotonic coating 24 at one end of test panel 20 isnon-hiding, i.e., one can clearly see the underlying patterns 22A and22B (no hiding) and position P_(n) at the other end of test pattern 20is completely hiding, i.e., one cannot see the underlying patterns 22Aand 22B (total hiding). Means 106 are conventional items, such as timersfor controlling exposure, switches, and aperture control needed toobtaining the desired degree of control of images of target area P₁ toP_(n).

Typically, the intensities Ir₁ to Ir_(n), Ig₁ to Ig_(n) and Ib₁ toIb_(n) of light portions 30 and intensities dr₁ to dr_(n), dg₁ to dg_(n)and db₁ to db_(n) of dark portions 32 and coating thicknesses X₁ toX_(n) that correspond to target areas P₁ to P_(n) are stored incomputing device 14. As shown in FIG. 2, if desired, device 1 caninclude a display device 15, such as a computer monitor for viewingimages of light portions 30 and dark portions 32 target areas P₁ toP_(n).

In use, step (i) of a method of characterizing hiding of a coatingcomposition includes applying a monotonic layer of the coatingcomposition over test pattern 22 affixed to hiding test panel 20 toproduce monotonic coating 24 thereon. The process for applying such amonotonic layer is well known. Typically, a coating composition, such asan automotive paint is successively sprayed in ever thicker layers fromone end of hiding panel 20 to the other end. Upon cure, a monotoniccoating 24 is produced on panel 20.

Step (ii) of the method of characterizing hiding of a coatingcomposition includes sequentially uniformly illuminating target areas P₁to P_(n) of monotonic coating 24, wherein each target area includeslight portion 30 and dark portion 32. Typically, light source 4 isadjusted to uniformly illuminate target area being analyzed. Thedistance from light source 4 to panel 20 is adjusted typically with setscrews (not shown) for maximum obtainable light intensity anduniformity. If needed, the intensity of light source 4 can be alsoadjusted by conventional means, such as a rheostat. As shown in FIG. 2,most of the adjustments are preferably programmed through computingdevice 14. However, it is within the contemplation of the invention touse separate automated or manual conventional means for accomplishingthe uniform illumination of target areas P₁ through to P_(n).

Step (iii) of the method of characterizing hiding of a coatingcomposition includes sequentially directing reflections of the targetareas P₁ to P_(n) to photosensitive device 8 for acquiring:

-   -   (a) intensities Ir₁ to Ir_(n), Ig₁ to Ig_(n) and Ib₁ to Ib_(n)        of light portions 30 of the areas P₁ to P_(n), and    -   (b) intensities dr₁ to dr_(n), dg₁ to dg_(n) and db₁ to db_(n)        of the dark portions 32 the areas P₁ to P_(n).

The forgoing sequential measurements are accomplished by using firstmotion translating system 6.

Step (iv) of the method of characterizing hiding of a coatingcomposition includes sequentially measuring applied measured thicknessesX₁ to X_(n) of monotonic coating 24 at locations 34 that respectivelycorrespond to the target areas P₁ to P_(n). The forgoing sequentialmeasurements are accomplished by using second motion translating system10.

Step (v) of the method of characterizing hiding of a coating compositionincludes sequentially directing reflections of the target areas P₁ toP_(n) to photosensitive device 8 for acquiring sequentially computingmeasured Y₁ to Y_(n) at the target areas P₁ to P_(n) by using theformula:

[(Ir_(i)−dr_(i))²+(Ig_(i)−dg_(i))²+(Ib_(i)−db_(i))²]^(0.5)  (1)

wherein i ranges from 1 to n, and the measured Y₁ to Y_(n) are measuredΔRGBs. The aforedescribed steps (i), (ii), (iii), (iv) and (v) arefurther explained through Table 1 below, which, by way of example, showsone illustration of measuring color differences (ΔRGB) Y₁ to Y_(n) attarget areas P₁ to P_(n) by using apparatus 1 of the present invention.Color intensities Ir₁ to Ir_(n), Ig₁ to Ig_(n) and Ib₁ to Ib_(n) oflight portions 30 of areas P₁ to P_(n), and intensities dr₁ to dr_(n),dg₁ to dg_(n) and db₁ to db_(n) of dark portions 32 of areas P₁ to P_(n)on monotonic coating 24 in red, green and blue colors, respectively weremeasured by using appropriate color filters on photosensitive system 8and ΔRGBs were calculated by means of the aforedescribed equation (1).Coating thicknesses X₁ to X_(n) corresponding to target areas P₁ toP_(n) were measured by using coating thickness detector 12 of apparatus1. The automotive applied on panel 20 was Imron® sparkling blueautomotive paint supplied by Dupont Company of Wilmington, Del.

TABLE 1 Corresponding Coating thickness Color Target Areas P₁ toCorresponding X₁ to X_(n) of coating difference P_(n) coating 24 in cm24 in microns ΔRGBs P₁  0.64 31.75 52.16 P₂  1.91 32.00 46.04 P₃  3.1834.54 38.07 P₄  4.45 34.29 30.3 P₅  5.72 39.12 28.53 P₆  6.99 41.6623.36 P₇  8.26 40.13 16.63 P₈  9.53 42.67 16.93 P₉  10.80 46.99 17.85P₁₀ 12.07 54.10 7.48 P₁₁ 13.34 58.93 10.46 P₁₂ 14.61 60.20 3.23 P₁₃15.88 58.17 5.28 P₁₄ 17.15 59.18 4.44 P₁₅ 18.42 61.21 5.64 P₁₆ 19.6967.31 2.86 P₁₇ 20.96 71.12 1.77 P₁₈ 22.23 73.91 1.55 P₁₉ 23.50 75.442.64 P₂₀ 24.77 75.18 2.26 P₂₁ 26.04 78.49 3.17 P₂₂ 27.31 86.36 2.16

FIG. 9 shows a graphic representation of data in Table 1. As notedearlier, target area P₁ has the most contrast between light portion 30and dark portion 32, i.e., a portion of monotonic coating 24 with leastthickness (maximum ΔRGBs) and P_(n), wherein n equals 22 has the leastcontrast between light portion 30 and dark portion 32, i.e., a portionof monotonic coating 24 with most thickness (minimum ΔRGBs). As thecoating thickness of coating 24 on hiding pattern 22 increases from P₁to P_(n), the color difference (ΔRGB) between the light portion 22A anddark portion 22B hiding pattern 22 on panel 20 decreases, and becomesless visible to the human eye. At a certain coating thickness somewherebetween P₁ and P_(n), the difference is no longer discernable to thehuman eye. That is the thickness at which the hiding occurs for thatparticular coating composition and such a thickness is called a “hidingthickness” for that particular coating composition. For different colorsand finishes the point at which the color difference is no longerdiscernable varies. As a result, determining a hiding thickness from themeasured ΔRGB data is more complex than simply measuring the colordifference that falls below a single threshold ΔRGB value.

Step (vi) of the method of characterizing hiding of a coatingcomposition includes storing on computing device 14 hiding datacomprising the measured Y₁ to Y_(n) and the applied thicknesses X₁ toX_(n).

Step (vii) of the method of characterizing hiding of a coatingcomposition includes determining a threshold measured Y_(th) by usingthe formula:

measured Y _(th)=Log_(e) (measured Y _(max)),  (2)

the measured Y_(max) being the maximum value within the range of themeasured Y₁ to Y_(n).

Step (viii) of the method of characterizing hiding of a coatingcomposition includes sequentially comparing the measured Y₁ throughY_(n) to identify first measured Y_(q) that is less than measured Y_(th)wherein q falls within said range 1 to n.

Step (ix) of the method of characterizing hiding of a coatingcomposition includes computing a ratio (q/n) to classify the hidingdata, wherein the hiding data is classified as:

-   -   (e) type 1 hiding data when the ratio is in the range of 0.01 to        less than 0.25,    -   (f) type 2 hiding data when the ratio is in the range of 0.25 to        less 0.35,    -   (g) type 3 hiding data when the ratio is in the range of 0.35 to        less 0.50, or    -   (h) type 4 hiding data when the ratio is in the range of 0.50 to        1.00.

As provided in the aforementioned step (ix) above, the method of thepresent invention first classifies the measured ΔRGB data and thenutilizes fitting functions to determine the hiding thickness. One canreadily observe from FIG. 9 that individual measurements do not fall ona smooth curve, which results from instrumental and measurements errorsthat are inherent in typical measurement devices. Therefore, a needexists to develop a fitting model that would mathematically permitfitting the measured data on to a smooth curve. The present processapplies a novel process to attain such a form fitting objective. Thus,in the aforementioned step (vii) a measured threshold Y_(th) is firstdetermined and then in the aforementioned step (viii) a first measuredY_(q) is identified by sequentially comparing the measured Y₁ thoroughto Y_(n) to locate a first Y₁ that has a value less than the measuredthreshold Y_(th), which is the first measured Y_(q). Once the value “q”is identified then in the foregoing step (ix) a ratio of (q/n) iscomputed to classify the hiding data as type 1, wherein (q/n) is in therange of from 0.01 to less than 0.25; type 2, wherein (q/n) is in therange of from 0.25 to less than 0.35; type 3 wherein (q/n) is in therange of from 0.35 to less than 0.50; or type 4 wherein (q/n) is in therange of from 0.50 to less than 1.00. In the foregoing classificationstep (ix) hiding data is classified on the basis of a degree ofsteepness or shallowness of the slope of the measured hiding data. Byway of example, the measured hiding data in FIG. 9 shows a shallow slopeand q/n is 0.55. Thus, such hiding data would be classified as type 4hiding data. By way illustration FIG. 10 shows how various types ofhiding data, i.e., types 1, 2, 3 and 4 would typically appear in agraphical format.

Step (x) of the method of characterizing hiding of a coating compositionincludes selecting one or more fitting equations applicable for the type1 hiding data, type 2 hiding data, type 3 hiding data or type 4 hidingdata, wherein the fitting equations define a relationship between (y)and (x), the (y) being a fitted color difference that corresponds to the(x), which is a fitted thickness on a fitted curve generated by the oneor more fitting equations. Once the hiding data is classified, in theforegoing step (x), a fitting equation that is most applicable for thetype of hiding data (type 1, type 2, type 3 or type 4) is selected froma library of various stored fitting equations.

When the hiding data are the type 4 hiding data, the fitted equation isof the formula:

y=a+bexp(−c*x)  (3)

wherein a, b and c are fitting parameters. By way of example, FIG. 11having steep slope illustrates the fitted curve obtained by using theforegoing equation 3 wherein the q/n is 0.27 and thus classified as atype 2. In FIG. 11 fitting parameter ‘a’ is the value of ‘y’ in equation3 at an asymptote point whereas Y_(max) would be the highest colordifference ΔRGB. In FIG. 11, one can readily observe the fitted curvethat would be generated by the fitting equation 3.

The fitting parameters a, b and c of the foregoing fitting equation 3are determined by the steps comprising:

(a) randomly assigning numerical values a¹, b¹ and c¹ to the fittingparameters;

(b) inserting the measured X₁ to the X_(n) into the fitting equation (3)to compute initial fitted y₁ ¹ to y_(n) ¹ generated by the fittingequation (3) inserted with the randomly assigned numerical values a¹, b¹and c¹;

(c) determining error ε between the initial fitted y₁ ¹ to y_(n) ¹ andthe measured Y₁ to Y_(n) by using the equation of formula:

-   -   n

ε=Σ(Y _(i) −y _(i) ¹)², and  (4)

(d) deriving the fitting parameters a, b and c by iteratively modifyingthe a¹, b¹ and c¹ by means of error minimizing equations until a changein the error ε between two successive iterations ranges from 1 to 10⁻¹⁰.

The aforementioned minimizing equations for the type 4 hiding data areof the formulas:

-   -   n

a ^((m+1)) =a ^(m) +k ₁Σ(Y _(i) −y _(i) ^(m)),  (5)

-   -   n

b ^((m+1)) =b ^(m) +k ₂Σ(Y _(i) −y _(i) ^(m))exp(−c ^(m) X _(i)),  (6)and;

-   -   n

c ^((m+1)) =c ^(m) −k ₃ b ^(m) ΣX _(i)(Y _(i) −y _(i) ^(m))exp(−c ^(m) X_(i)),  (7)

wherein when m=1, the randomly assigned numerical values are a¹, b¹ andc¹, the m and m+1 being consecutive iterations, and wherein:

-   -   the k₁ is 1/(22)²,    -   n    -   the k₂ is 1/[Σ exp(−c^(m)X_(i))]² and    -   n    -   the k₃ is 1/[−bΣX_(i) exp(−c^(m)X_(i))]².

When the hiding data are the type 1 hiding data, the fitted equation isof the formula:

x=−(1/c)log_(e)[(y−a)/b]  (8)

wherein a, b and c are fitting parameters, wherein the fitting parameter‘a’ ranges from a lowest the measured value Y_(lw) to an average,preferably an arithmetic average of measured the Y_(q) to Y_(n) and thefitting parameters ‘b’ and ‘c’ are determined by the steps comprising:

(a) randomly assigning numerical values b¹ and c¹ to the fittingparameters;

(b) inserting the measured Y₁ to the Y_(q−1) into the fitting equation(8) to compute initial fitted x₁ ¹ to x_(q−1) ¹ generated by the fittingequation (8) inserted with the randomly assigned numerical values b¹ andc¹;

(c) determining error ε between the initial fitted x₁ ¹ to x_(q−1) ¹ andthe measured X₁ to X_(q−1) by using the equation of formula:

-   -   q−1

ε=Σ(X _(i−x) _(i) ¹)², and  (9)

(d) deriving the fitting parameters b and c by iteratively modifying theb¹ and c¹ by means of error minimizing equations until a change in theerror ε between two successive iterations ranges from 1 to 10⁻¹⁰.

The aforementioned minimizing equations for the type 1 hiding data areof the formulas:

-   -   q−1

b ^((m+1)) =b ^(m)+(k ₂ /b ^(m) c ^(m))Σ(X ^(i) −x _(i) ^(m)) and  (11)

-   -   q−1

c ^((m+1)) =c ^(m) −k ₃/(c ^(m))²Σ log_(e){(Y _(i) −a)/(b ^(m))}(X _(i)−x _(i) ^(m))  (12)

wherein when m=1, the randomly assigned numerical values are b¹ and c¹,the m and m+1 being consecutive iterations, and wherein:

-   -   q−1

k ₂={1/(Σ(1/b ^(m) c ^(m)))}² and

-   -   n

k ₃=[Σ{(c ^(m))²/(log_(e)((Y _(i) −a)/(b ^(m)))}]².

When the hiding data are the types 2 or 3 hiding data, the fittedequation is of the formula:

x=−(1/c ^(#))log_(e)[(y−a)/b ^(#)], and  (13)

wherein for the measured Y₁ to Y_(n) the fitted equation is of theformula:

y=a+b ^(@)exp(−c^(@) *x)  (14)

wherein a, b^(#), and c^(#) and c^(@) are fitting parameters, whereinfitting parameter ‘a’ ranges from a lowest the measured value Y_(lw) toan average, preferably an arithmetic average, of measured the Y_(q) toY_(n); and the b^(#) and c^(#) parameters of the equation (13) aredetermined by the steps comprising:

(a) randomly assigning numerical values b¹ and c¹ to the fittingparameters;

(b) inserting the measured Y₁ to the Y_(q−1) into the fitting equation(13) to compute initial fitted x₁ ¹ to x_(q−1) ¹ generated by thefitting equation (13) inserted with the randomly assigned numericalvalues b¹ and c¹;

(c) determining error ε between the initial fitted x₁ ¹ to x_(q−1) ¹ andthe measured X₁ to X_(q−1) by using the equation of formula:

-   -   q−1

ε=Σ(X _(i) −x _(i) ¹)², and  (15)

(d) deriving the fitting parameters b^(#) and c^(#) by iterativelymodifying the b¹ and c¹ by means of error minimizing equations until achange in the error ε between two successive iterations ranges from 1 to10⁻¹⁰.

The aforementioned minimizing equations for the types 2 or 3 hiding dataare of the formulas utilizing equation (13) are:

-   -   q−1

b ^(#(m+1)) =b ^(#m)+(k ₂ /b ^(#m) c ^(#m))Σ(X _(i) −x _(i) ^(m))and  (16)

-   -   q−1

c ^(#(m+1)) =c ^(#m) −k ₃/(c ^(#m))²Σ log_(e){(Y _(i) −a)/(b ^(#m))}(X_(i) −x _(i) ^(m))  (17)

wherein when m=1, the randomly assigned numerical values are b¹ and c¹,the m and m+1 being consecutive iterations, and wherein:

-   -   q−1

k ₂={1/(Σ(1/b ^(#m) c ^(#m)))}² and

-   -   n

k ₃=[Σ{(c ^(#m))²/(log_(e)((Y _(i) −a)/(b ^(#m)))}]².

In aforementioned equation (14), the b^(@) and c^(@) parameters aredetermined by the steps comprising:

(a) assigning numerical values b^(#) and c^(#) to the fittingparameters;

(b) inserting the measured X₁ to the X_(n) into the fitting equation(14) to compute initial fitted y₁ ^(#) to y_(n) ^(#) generated by thefitting equation (14) inserted with the assigned numerical values b^(#)and c^(#);

(c) determining error ε between the initial fitted y₁ ^(#) to y_(n) ^(#)and the measured Y₁ to Y_(n) by using the equation of formula:

-   -   n

ε=Σ(Yi−yi ^(#))², and  (18)

(d) deriving the fitting parameters b^(@) and c^(@) by iterativelymodifying the b^(#) and c^(#) by means of error minimizing equationsuntil a change in the error ε between two successive iterations rangesfrom 1 to 10⁻¹⁰.

The aforementioned minimizing equations for the types 2 or 3 hiding dataare of the formulas utilizing equation (14) are:

-   -   n

a ^((m+1)) =a ^(m) +k ₁Σ(Y _(i) −y _(i) ^(m)),  (19)

-   -   n

b ^((m+1)) =b ^(m) +k ₂Σ(Y _(i) −y _(i) ^(m))exp(−c ^(m) X _(i)),  (20)and;

-   -   n

c ^((m+1)) =c ^(m) −k ₃ b ^(m) Σx _(i)(Y _(i) −y _(i) ^(m))exp(−c ^(m) X_(i)),  (21)

wherein when m=1, the randomly assigned numerical values are a¹, b¹ andc¹, the m and m+1 being consecutive iterations, and wherein:

-   -   the k₁ is 1/(22)²,    -   n    -   the k₂ is 1/[Σ exp(−c^(m)X_(i))]² and    -   n    -   the k₃ is 1/[−b Σ X_(i) exp(−c^(m)X_(i))]².

Step (xi) of the method of characterizing hiding of a coatingcomposition includes fitting said one or more selected fitting equationsto match paired measured (X₁, Y₁) to paired measured (X_(n), Y_(n)),wherein the fitted curve has a fitted baseline value of y_(b) at anasymptote of said fitted curve.

Step (xii) of the method of characterizing hiding of a coatingcomposition includes selecting a fitted threshold value y_(th) above thefitted baseline value y_(b), wherein the fitted threshold value y_(th)is suited for the type 1 hiding data, type 2 hiding data, type 3 hidingdata or type 4 hiding data.

In the foregoing step (xii) when the hiding data are the type 3 or type4 hiding data, the fitted threshold value y_(th) is 1 when (Y_(max)−a)is either more than 15 or the Y_(max) is in the range of 10 to 255. Inall other cases when (Y_(max)−a) is either less than 15 and said Y_(max)is less than 10, the fitted threshold value y_(th) is expressed by theformula:

0.1*(Y_(max)/a)*Log 10{(Y_(max)−a)/(a+1)}  (22)

provided the value expressed by the formula (19) is more than 0.3 (asshown in FIG. 11 y_(th) is about 1.5); or the fitted threshold valuey_(th) is 0.3 if value expressed by the formula (22) is equal to or lessthan 0.3.

In the foregoing step (xii) and as shown in FIG. 12 when the hiding dataare the type 1 or type 2 hiding data, the fitted threshold value y_(th)is determined by the steps comprising:

(a) computing slope S_(x) by using the following formula:

S _(x) =d[ Log {y}]/dx={−b*c*exp(−cx)}/[ Log {a+b*exp(−cx)}];  (23)

(b) selecting slope S_(max) at the measured thickness X₁;

(c) selecting first measured thickness X_(v) at less than half of theslope S_(x);

(d) establishing a first range of the fitted y_(v−6) to y_(v−1) on afitted log₁₀ curve that correspond to measured thicknesses ranging fromX_(v−6) to X_(v−1) and a second range of the fitted y_(v) to the y_(n)on the fitted log₁₀ curve that correspond to measured thicknessesranging from the X_(v) to X_(n);

As shown in FIG. 13 in step (e) inserting the fitted y_(v−6) to y_(v−1)and the from X_(v−6) to X_(v−1) of the first range in the followingformula:

Log₁₀ y=w ₁ x+g ₁  (24)

wherein the w₁ is a slope of a first straight line computed by theformula (24) and the g₁ is a value of y when x=0 in the formula (24);

(f) inserting the fitted y_(v) to y_(n) and the from X_(v) to X_(n) ofthe second range in the following formula:

Log₁₀ y=w ₂ x+g ₂  (25)

wherein the w₂ is a slope of a second straight line computed by theformula (22) and the g₂ is a value of y when x=0 in the formula (25);and

(g) computing a point of intersection of the first and the secondstraight lines obtained though the formula:

Log₁₀ y _(th)=(w ₂ g ₁ −w ₁ g ₂)/(w ₂ −w ₁)  (26).

Step (xii) of the method of characterizing hiding of a coatingcomposition includes locating a hiding thickness x_(h) of the coatingcomposition that corresponds to the fitted threshold value y_(th) on thefitted curve. One can readily note in FIG. 13 the point x_(h).

The coating composition suitable use in the present method can be arefinish automotive paint, OEM automotive paint, architectural paint, oran industrial paint.

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 21. An apparatus for characterizing hiding of a coatingcomposition, said apparatus comprising: (i) a light source forilluminating target areas P₁ to P_(n) of a hiding test panel at adesired angle of incidence and light source intensity wherein each saidtarget area comprises a light portion and a dark portion, said hidingtest panel having a monotonic coating from said coating compositionapplied thereon; (ii) a first motion translating system affixed to a bedof said apparatus, said first motion translating system comprising afirst movable stage and a first mechanism for translating said firstmovable stage; (iii) a fixture affixed to said movable stage to positionsaid hiding test panel thereon; (iv) a photosensitive system affixed tosaid bed of said apparatus, said photosensitive system being positionedto receive reflections of said light portion and said dark portion ofeach said target area of said hiding test panel; (v) a second motiontranslating system affixed to said bed of said apparatus, said secondmotion translating system comprising a second movable stage and a secondmechanism for translating said second movable stage in a directionperpendicular to that of said first movable stage; (vi) a coatingthickness detector affixed to said second movable stage for measuringthicknesses X₁ to X_(n) at locations that respectively correspond tosaid target areas P₁ to P_(n); and (vii) a computing device connected tosaid light source, said photosensitive system, said first and secondmotion translating systems and said coating thickness detector to directsteps performed by said light source, said photosensitive system, saidfirst and second motion translating system and said coating thicknessdetector in accordance with a computer readable program code meansstored in said computing device.
 22. The apparatus of claim 21 whereinsaid computer readable program code means comprise: (a) means forcontrolling said first motion translating system to sequentially directsaid reflections of said light portion and said dark portion of saidtarget areas P₁ to P_(n) of said monotonic coating to saidphotosensitive system to acquire intensities Ir₁ to I_(rn), Ig₁ toIg_(n) and Ib₁ to Ib_(n) of said light portions of said areas P₁ toP_(n), and intensities dr₁ to dr_(n), dg₁ to dg_(n) and db₁ to db_(n) ofsaid dark portions said areas P₁ to P_(n); and (b) means for controllingsaid second motion translating system to sequentially direct saidcoating thickness detector for measuring said thicknesses X₁ to X_(n) atlocations that respectively correspond to said target areas P₁ to P_(n).23. The method of claim 22 wherein said computer readable program codemeans further comprise means for controlling time of exposure of a photosensitive surface in said photosensitive system to said reflections ofsaid light portion and said dark portion of said target area P₁ toattain highest contrast between said light portion and said dark portionof said target area P₁.
 24. The apparatus of claim 21 wherein saidintensities of said light and dark portions of each said target areasand said coating thickness X₁ to X_(n) that corresponds to said targetareas P₁ to P_(n) are stored in said computing device.
 25. The apparatusclaim 21 further comprising a display apparatus for viewing images ofsaid dark and light portions of said target areas P₁ to P_(n).
 26. Theapparatus of claim 21 wherein said photosensitive system is positionedat 90 degrees to said hiding test panel.
 27. The apparatus of claim 21or 26 wherein said light source is positioned at an angle ranging from 5degrees to 60 degrees to said hiding test panel.